There is an increasing need to remove, from exhaust gas, particulate matter included in the exhaust gas discharged from an internal combustion engine such as an engine for an automobile, an engine for a construction machine or a stational engine for an industrial machine, other combustion devices and the like in consideration of an influence on an environment. In particular, a regulation concerning the removal of the particulate matter (also referred to as the PM) discharged from a diesel engine tends to be strengthened worldwide. From such a situation, a diesel particulate filter (DPF) for trapping and removing the PM attracts attention.
Examples of the DPF include a honeycomb filter provided with porous partition walls which separate and form a plurality of cells functioning as fluid flow paths. Predetermined cells each having one end opening on one side and the other end plugged on the other side and the remaining cells each having one end plugged on one side and the other end opened on the other side are alternately arranged. Fluid (the exhaust gas) which has flowed into one open end of the honeycomb filter where the predetermined cells open passes through the partition walls and is discharged as the transmitted fluid toward the remaining cells. Furthermore, the exhaust gas is discharged from the other end of the honeycomb filter where the remaining cells open, and the PM is trapped and removed from the exhaust gas. In a filter (a wall flow type filter) such as this honeycomb filter having a structure in which the exhaust gas passes through the porous partition walls, a large filtration area is obtained, so that a filtration flow rate (the partition wall passing flow rate) can be lowered, and pressure loss is decreased. Furthermore, the filter has a comparatively satisfactory particulate matter trapping efficiency.
It is to be noted that examples of prior documents having the same problem as that described later of the present invention include Patent Documents 1 to 3. In Patent Documents 1, 2, a technology for providing fiber layers as the surface layers of the partition walls is disclosed, and the shape of a material for preparing the surface layer is defined. Moreover, in Patent Document 3, a technology of filling pores in the partition walls with fibers or whiskers is disclosed.
Patent Document 1: JP-A-2004-216226
Patent Document 2: JP-A-6-33734
Patent Document 3: Japanese Patent No. 2675071